1. Field of the Invention
The invention relates to technical woven fabrics of high thermal stability which have an air permeability set to a specific value via the linear density-dependent choice of the numbers of threads and via a relaxation in a wet process and for a side airbag manufactured from this fabric.
2. Description of the Prior Art
Technical fabrics for some applications are required to possess not only a specific air permeability but also in particular good heat resistance. This is true, for example, of fabrics for hot gas filtration and in particular for airbag fabrics, especially for side airbags, installed at the side of the vehicle such as in a door or seat.
The woven fabrics used for airbags have to meet a number of very important requirements. Hitherto airbags were mainly manufactured using coated fabrics of very low air permeability. Lately, airbag manufacturers are increasingly using uncoated fabrics. With these fabrics the air permeability is an especially important property. In the contact part of the airbag, i.e. the part towards which the generator-produced gas flows in the event of the airbag function being deployed, it should be very low at &lt;10 l/dm.sup.2 xmin. However, in order that the gas can slowly escape again in a controlled fashion, the side parts of the airbag have to be more air-permeable. These filter portions of the fabric are required to have air permeabilities between 40 and 100 l/dm.sup.2 xmin, depending on the vehicle type. This kind of airbag, consisting of two fabrics with different air permeabilities, is required for driver airbags and for passenger airbags which are, when expanded, in front of the car driver and the passenger.
In recent times a further type of airbag has become more and more the focus of attention. This is the side airbag typically installed in the door of the car or in the seat. This airbag is expanded in the case of a collision at the side of the vehicle and protects the driver or the passenger against injuries inflicted by the side of the car and penetrating metal elements. Side airbags are manufactured of only one kind of fabric with a low air permeability of &lt;20 l/dm.sup.2.min, preferred &lt;10 l/dm.sup.2.min.
As well as the aforementioned, specific air permeability, airbag fabrics are required to have good foldability and a low fabric thickness to minimize the space required when the airbag is accommodated in the vehicle, for example inside the steering wheel. To ensure optimum functioning of the airbag system, the mass of fabric should be as low as possible. A low fabric mass is especially beneficial in the event of out-of-position accidents, when the driver or front seat passenger is not in his or her normal seated position at the time of the collision. The momentum of impact of the explosively expanding airbag on the vehicle occupant decreases with decreasing fabric mass and hence a lower fabric mass makes possible softer cushioning of the vehicle occupant by the airbag. A further requirement is that the airbag fabric should have a high strength, which should be the same in both directions of the weave.
The special problem with airbags, particularly side airbags, is thermal failure of the fabric. By this is meant the softened or fully molten areas of the fabric through which the gas in the airbag can escape uncontrollably and which may make possible the emergence into the passenger compartment of glowing particles which may cause burn injuries to the vehicle occupants. These hot and glowing particles are formed in the course of the explosive ignition of the gas generator. The gas carries the particles into the airbag where they can soften or burn through individual areas of the airbag fabric.
A particular problem area of the driver and passenger airbag manufactured of two fabrics sewn together is the seam region of the airbag. Owing to the stresses which these areas are subjected to in the course of the expansion of the airbag, there is a danger that hot gases will increasingly flow through the needle holes and thus subject the airbag fabric to particular thermal stresses in these locations.
To solve this problem DE-A-21 59 449 proposes absorbing the heat by coating the inner surface of the airbag with a film or by introducing a liner into the airbag. As well as the much higher manufacturing costs of this process, airbags of this kind have the serious disadvantage of needing more space on account of the greatly increased fabric thickness due to the coating with a film or an appropriate liner, and hence of a high packing volume. In addition, closely packed films--unavoidable in the storage of the folded airbag in the steering wheel of a vehicle--tend to bond. Bonding can be controlled by sprinkling with talcum powder. In the event of the airbag function being deployed this powder then leads to a dense fog in the passenger compartment, since it passes into said compartment together with the gas escaping from the airbag. This represents not only an appreciable general nuisance to the vehicle occupants but also greatly impairs their vision.
U.S. Pat. No. 3,814,458 proposes improving the absorption of heat with an aluminized neoprene coating. This is likewise subject to the aforementioned disadvantages such as high airbag manufacturing costs, the great deal of space required for accommodating the airbag in the vehicle and an appreciable nuisance to the vehicle occupants in the event of the airbag function being deployed owing to the talcum powder required for sprinkling on the coating.
DE-C-36 44 554 proposes sewing a filter fabric of aromatic polyamide fibers into the airbag. This type of fiber has good thermal stability and thus offers adequate protection against thermal failure of the fabric. However, protection against glowing particles is produced only in respect of the area of the filter fabric, which is comparatively small compared with the total area of the airbag. The other fabric parts of the airbag are left unprotected. A further disadvantage of the airbags of the embodiment proposed are the high manufacturing costs and a multimaterial system which complicates recycling.
Airbag fabrics are normally manufactured using polyester or polyamide fibers. The former fibers are customarily based on polyethylene terephthalate, whilst the latter fibers based on polyamide-6.6 (polyhexamethyleneadipamide) are preferred. Neither polyester nor polyamide-6.6 fibers ensure adequate prevention of particle burn-through. For this reason it has been considered to manufacture airbag fabrics from fibers of higher thermal resistance. However, fabrics made from these fibers do not meet all the requirements of uncoated airbag fabrics such as high strength and suitable shrinkage properties which are necessary for manufacturing an uncoated, dense woven fabric of low air permeability in order that additional costly finishing operations may be dispensed with. Moreover, high temperature resistant fibers have the disadvantage of a very high raw material price.
JP-A-01/104 848 and JP-A-64/041 438 mention, in addition to a large number of polymers for synthetic fibers suitable for manufacturing coated airbag fabrics, polymers which result in fibers of high thermal stability. However, neither of these cited patent publications provides any indication as to how an uncoated airbag fabric of good thermal stability is to be manufactured. JP-A 01 - 041 438 only discloses coated fabrics. In JP 01 - 104 848 ('848) coated and uncoated fabrics are disclosed. To obtain fabrics with a low air permeability, '848 requires coating. The air permeability of the uncoated fabric of Embodiments of '848 is calculated to be 62 l/dm.sup.2.min under a test differential pressure of 500 Pa. This is the lowest air permeability obtained with uncoated fabrics taught by '848. To obtain an air permeability as e.g. required for side airbags, '848 requires coating (see comparison example No. 3) in which the air permeability achieved is calculated to be &lt;5 l/dm.sup.2.min).
It is, therefore, the object of the present invention to make available a technical woven fabric which has a specifically set air permeability, which is uncoated and which meets all the other requirements of woven fabrics of this kind such as, for example, high strength and good foldability and which, moreover, offers good protection against thermal failure on account of an adequate heat capacity and which can be produced at approximately the same manufacturing costs as the previously employed fabrics made of polyamide 6.6 or polyester yarns. Moreover, this object is to be achieved with a recyclable single-material system.
It has now been found, surprisingly, that this object is achieved when the fiber material used is polytetramethyleneadipamide (nylon 4.6) which has a unique suitability for use in fabrics for airbag construction, especially for side airbags. There has been a long felt but unsolved need for an uncoated fabric to replace coated fabric and still have the necessary thermal stability and protection against thermal failure, strength, low air permeability and foldability. The present invention fulfills that need.